This invention relates to solder glasses and more particularly to such glasses which become molten at from about 388xc2x0 C. to about 466xc2x0 C. and can be employed with quartz-to-metal seals in electrical devices.
Certain electrical devices, e.g., tungsten halogen and metal halide lamps, employ envelopes comprising a high percentage of silica, such as fused silica, fused quartz or Vycor, the latter being a 96% silica glass. To achieve a hermetic seal between the glass and the lead-in conductors of the electrical device it is common practice to employ a pinch seal. The lead-in conductors generally comprise a very thin foil portion of molybdenum which forms the actual hermetic seal and an outer lead-wire of a refractory metal which can also be molybdenum. One end of the lead-wire is attached to the foil and the other end extends exteriorly of the pinch. This relatively heavy outer lead-wire does not form a hermetic seal with the quartz because of its relatively large diameter and the difference in thermal expansion coefficients between the two. In actual practice, because the formation of the pinch seal does not allow the quartz to flow completely around and against the full periphery of the outer lead-wire, a small capillary passage is left therebetween. The thin foil can thus be exposed to atmospheric oxygen via the capillary passages. At elevated temperatures, say above 350xc2x0 C., oxidation of the foil can occur, resulting in breakage of the electrical connection to the outer lead-wire.
It has been suggested that this problem can be reduced by filling the capillary with a solder glass which becomes molten when the device is operated, thus forming a molten seal. Specifically, a lead borate glass has been suggested; however, the use of such a glass requires the use of platinum or platinum-clad lead-wires since lead borate attacks molybdenum. Further, U.S. Pat. No. 3,588,315 suggests binary glasses such as antimony borate and ternary glass compositions of antimony borate with the addition of small amounts of molybdenum trioxide or tungsten trioxide.
Additionally, U.S. Pat. Nos. 4,492,814; 4,521,641, and 4,493,944, disclose improvements in the antimony borate systems wherein the glasses include, respectively, 5% of: Bi2O3; V2O3; and PbO. All of the latter glasses have melting points at about 350xc2x0 C.
Recently, it has been discovered that lamps which operate at higher wattages and which have higher operating seal temperatures (i.e., above 400xc2x0 C.) have not been adequately protected from moly-foil oxidation, resulting in premature lamp failures.
It is, therefore, an object of the invention, to obviate the disadvantages of the prior art.
It is another object of the invention to provide a solder glass that is suitable for use with electrical devices having seal temperatures in the neighborhood of 400xc2x0 C.
It is yet another object of the invention to provide such a glass which is not deleterious to molybdenum.
These objects are accomplished, in one aspect of the invention, by a solder glass comprising, by weight, about 60 to 67% Sb2O3; about 27 to 32% B2O3; and from greater than 0 to 10% ZnO. This glass can have a softening point between about 388xc2x0 C. with 1% ZnO to about 466xc2x0 C. at 10% ZnO. In a preferred composition, the glass comprises, by weight, about 63.6% Sb2O3; about 29.4% B2O3; and about 7% ZnO and has a softening point of about 440xc2x0 C.